1. Field of the Invention
The present invention relates to a toner for use in electrophotographic image forming methods, electrostatic recording methods and electrostatic printing methods.
2. Discussion of the Background
The electrophotographic image forming method typically includes forming an electrostatic latent image on a photoreceptor (an electrostatic latent image bearer); developing the electrostatic latent image with a developer including a toner to form a visible image (a toner image); and transferring and fixing the visible image onto a recording medium such as papers.
The methods of developing the electrostatic latent image are broadly classified to wet developing methods such as a cascade method, a magnetic brush method and a powder cloud method, and dry developing methods using a toner wherein a colorant such as carbon black is dispersed in a natural or synthesized resin. Currently, the dry developing methods are widely used.
As a fixing method used in the dry developing methods, a heat roller fixing method directly contacting a heating roller to the toner image upon application of pressure and fixing the toner image on the transfer material is widely used because the method has good heat efficiency and the heating roller can be downsized. Recently, the heat roller is required to consume less electric power for fixing to save energy.
In order to save energy, the fixer has been improved to further increase the heat energy efficiency, e.g., the heat roller has a thinner layer contacting a toner image and a much shorter warm-up time.
However, the heating roller has a smaller specific heat capacity, and a difference of temperature between a part a recording medium passes and a part the recording medium does not pass thereof becomes large. Accordingly, a melted toner adheres thereto, and after the heating roller makes one revolution, the melted toner adheres to a part of the transfer material, having no image, i.e., a hot offset problem tends to occur. Therefore, a toner is required to have hot offset resistance.
In addition, a heat energy applied to a toner tends to decrease as does in a low-temperature fixer and a high-speed copier for saving energy. A toner fixable at a low temperature typically includes a resin or a wax having a low softening point.
However, such a toner as is fixable at a low temperature is liable to be hardened, i.e., blocked, with other heats such as a heat of an apparatus including the toner or a heat when stored. Further, the toner is difficult to have a wide fixable temperature.
For the purpose of improving the low-temperature fixability of a toner, e.g., Japanese Laid-Open Patent Publication No. 62-63940 discloses a method of including a specific non-olefin crystalline polymer having a sharp melt profile in a binder resin of the toner, Japanese Patent No. discloses a method of including a crystalline polyester having a sharp melt profile therein and Japanese Laid-Open Patent Publication No. 2003-167384 discloses a toner including a crystalline resin and an amorphous resin incompatible with each other.
However, these methods cannot prepare a toner having sufficient low-temperature fixability, and when a glass transition temperature thereof is lowered too much, thermostable preservability thereof deteriorates. In addition, when the molecular weight thereof is decreased to lower the softening point too much, the hot offset occurs at a lower temperature. Therefore, a toner having both low-temperature fixability and thermostable preservability is difficult to prepare.
Because of these reasons, a need exists for a toner having good hot offset resistance, both low-temperature fixability and thermostable preservability, and producing high-quality images.